Anal sex within heterosexual relationships is usually underreported or not reported at all, yet is increasingly recognised as a potential mode of HIV transmission. Understanding the circumstances of anal sex is critical for trials that seek to assess the efficacy of microbicides. This article draws on qualitative data collected during a feasibility study for a clinical trial of microbicides in Soweto, South Africa. Focus groups of women enrolled in the feasibility study discussed the circumstances under which they and other women in the community had anal sex. Their narratives drew attention to the ambivalent meanings of anal sex; often regarded as a form of sexual coercion but also frequently as sexual pleasure. The article explores the reasons for these apparent contradictions.